1. FIELD OF THE INVENTION
This invention relates to a method of producing sodium percarbonate in the form of crystals or powder having a long shelf life or an excellent storage stability.
2. DESCRIPTION OF THE PRIOR ART
It is already known that inorganic peroxides which release active oxygen in an aqueous solution are used as a domestic or industrial bleaching agent or bleaching detergent composition. Among such inorganic peroxides, sodium perborate has a good stability during storage whether it is used as a bleaching agent or as an additive in a detergent composition as it does not substantially decompose during storage to lose available oxygen which is effective for the purpose of bleaching. However, it does not demonstrate its high bleaching effect unless it is used at a temperature over 60.degree. C., so that this substance cannot show a sufficient bleaching effect in a country where laundering is generally performed at a low temperature.
On the other hand, sodium percarbonate has a higher solubility in water than sodium perborate and is also easily dissolved in cold water. Sodium percarbonate has the formula: 2Na.sub.2 CO.sub.3.3H.sub.2 O.sub.2. It is generally prepared by reacting 2 mols of sodium carbonate with 3 mols of hydrogen peroxide. However, if sodium percarbonate is mixed in a powdery detergent composition or bleaching agent composition, it is decomposed too rapidly under the atmospheric conditions where the temperature exceeds 30.degree. C. and the humidity is higher than 80% RH, so that no satisfactory bleaching effect can be obtained in a high-temperature high-humidity atmosphere. Various attempts have been made for improving the stability during storage of sodium percarbonate, and there have been proposed the following processes for stabilizing sodium percarbonate:
(1) A method in which an inorganic salt known as a peroxide stabilizer, such as, for example, sodium silicate, magnesium silicate, magnesium oxide, sodium chloride or sodium sulfate, is added to sodium percarbonate. PA0 (2) A method in which an organic metal chelating agent of the water-soluble polyaminocarboxylic acid type, such as, for example, ethylenediaminetetraacetate, nitrilotriacetate or diethylenetriaminepentaacetate, is added to sodium percarbonate. PA0 (3) A method in which sodium percarbonate is granulated or coated with a water-soluble or water-insoluble or slightly water-soluble compound such as paraffin wax, polyethyleneglycol or alkanolamide.
These known methods, however, were not satisfactory, because they are insufficient with respect to their stabilizing effect on sodium percarbonate and a considerable decomposition of sodium percarbonate is caused during storage, or they are too complicated for practical use, or the water-solubility of sodium percarbonate is reduced remarkably in the process of coating the same.